Stereolithography is a relatively new technology linking the power of computer graphics to the rapid formation of a solid, shaped object. This technology uses UV laser to selectively solidify successive thin layers of a photo-curable resin and provides great economies for both the design lab and the modeling process. Since the invention of stereolithography in 1984, this technology has enabled a new method for rapid prototyping (RP), appearing in 1988, which allows designers to verify their product design at an early stage by using 3D representations for design review with sales, marketing and production engineers. However, many of these systems produce only fragile parts, which can only be used for form and fit analysis and are not suitable for any other function.
In 1995, a technology was developed to deposit metals onto the surfaces of non-metallic RP parts, particularly SLA parts. Of all the available metallization processes, such as electroless deposition, physical vapor deposition (PVD) and chemical vapor deposition (CVD), the former is the most applicable for SLA parts due to its possession of combined characteristics such as uniformity, simple operation, low cost and no damage to the substrate.
In order to apply electroless deposition to a non-catalytic surface, the surface must be subjected to a series of pre-treatment steps such as pickling, sensitization and activation. Of all the three pre-treatment processes, pickling is the most important process that provides the substrate with cavities, and in some cases a modified chemistry, which improve the adherence of coating. There are two ways to pickle the surface: sand blasting/glass beading, and chemical etching.
Sand blasting process uses a stream of high-pressure sand of certain size to bombard the substrate surface thereby creating cavities. The advantage of this process is its independence of substrate materials. However, sand blasting has disadvantages such as non-uniform surface preparation, lack of consistency (dependency of the operator's skill level), time consuming (particularly for large parts), and most importantly, inability to prepare features that the sand stream cannot access, such as deep cavities.
Chemical etching is a commonly known pickling process that has been successfully applied to numerous types of polymers such as ABS. The disadvantage of chemical etching is that the process is dependent on the composition of the substrate. Thus, not all types of polymers may be successfully chemically etched and a chemical etchant for one polymer may not work for another. SLA polymers have very different chemical compositions than other polymers, therefore it is not a priori obvious that a particular etching process for one polymer would be useful for SLA polymers. However, chemical etching has advantages over sand blasting, for example, uniformity, consistency, short preparation time for small and large parts, and ability to treat complex shapes. Therefore, it would be useful to have a chemical etchant for SLA polymers.
Currently, chemical etching of SLA parts in a coating process is not used commercially, though it is much needed. One process for chemically etching SLA parts was reported by Luan et al. in Applied Surface Science, 156: 26-38 (2000) and involves chromic acid. Chromic acid is environmentally unfriendly and the process is very complicated both in terms of the chemical composition and in terms of operation. Further, this process often damages the part. Furthermore, this process provides grooves and trenches in the surface of the SLA part but not much etching between the grooves and trenches. As a result, there is inconsistent coverage of the coating material on the SLA part, leading to weaker parts that are less useful in the rapid prototyping industry.
Other standard chemical etchants known in the art, for example H2SO4/HF used to etch epoxy polymers, do not work effectively for SLA polymers. Currently, there is no suitable way to effectively etch SLA polymers chemically.
There remains a need for a functional, simple and more environmentally friendly chemical etching process for coating of SLA polymers.